Low-Effluent Syngas Handling System

ABSTRACT

A system for processing a syngas stream including particulate matter, a combustible gas, and acid components is disclosed. The system includes a gasifier vessel configured to produce a raw syngas stream; a gas cooling apparatus configured to cool the raw syngas stream to produce a cooled syngas stream; an HCl and particulate removal apparatus configured to produce a reduced-HCl syngas stream; a first reheat apparatus configured to produce a first reheated syngas stream; a COS and HCN hydrolysis apparatus configured to produce a hydrolyzed syngas stream; an H 2 S removal apparatus configured to produce a reduced-H 2 S syngas stream; a second reheat apparatus configured to produce a second reheated syngas stream; an activated carbon bed apparatus configured to produce a polished syngas stream; and a compression and intercooling apparatus configured to compress and cool the polished syngas stream to produce a clean syngas stream.

FIELD OF THE INVENTION

The described embodiments include systems that relate to processing syngas produced in gasification reactors.

BACKGROUND

It is well-known that various types of reactors can be used to produce syngas from feed material such as municipal solid waste, biomass, or other feed materials. The syngas produced by such reactors may include numerous contaminants. Many existing syngas cleaning systems employ wet-based processes, such as wet scrubbers/quenchers, to remove certain contaminants (e.g. acid gases, particulates). Such processes often require a significant water input, and may generate a considerable amount of liquid waste (i.e. effluent) containing a variety of contaminants. These effluent streams generally require significant treatment prior to discharging to the environment, increasing the overall cost of the gasification plant. For example, US Patent Application Publication No. 2014/0252276 discloses a syngas clean-up system which preferably utilizes water-based scrubbing technology for chlorine, sulphur, and ammonia removal. Similarly, U.S. Pat. No. 8,980,204 describes a system for syngas treatment which utilizes wet-based processes for removal of particulates, tars, chlorine, and ammonia. In certain syngas cleaning applications, it may be desirable to minimize the water input requirements and/or the effluent discharge rate (e.g. in jurisdictions with low water availability and/or low-effluent regulations).

In most syngas cleaning systems, it is generally not possible to completely eliminate liquid discharge from the syngas cleaning system. For example, syngas produced in a gasifier vessel from municipal solid waste (MSW) usually contains more water than is acceptable for its ultimate end-use. Given this limitation, it may be preferable to minimize the contaminant levels within any liquid discharge stream, in order to minimize any water treatment requirements.

Syngas treatment systems using dry-based processes are known in the art, but employ expensive and/or less proven technology for contaminant removal. For example, U.S. Pat. No. 7,056,487 discloses a syngas cleaning system that may utilize zinc-based, iron-based, or copper-based sorbents for dry sulphur removal. These sorbents are relatively expensive, and their performance has not been extensively demonstrated at the commercial scale. Furthermore, such sorbents require relatively high temperatures to achieve sulphur reductions acceptable for end-use applications (e.g., U.S. Pat. No. 7,056,487 discloses a preferred operating temperature of about 1,000 degrees Fahrenheit). This high-temperature requirement demands temperature-resistant, and therefore costly, materials of construction. As another example, U.S. Pat. No. 6,090,356 describes a process for removal of acidic gases from syngas using a liquid solvent such as methanol or dimethyl ether of polyethylene glycol. While such a process may not generate effluent streams requiring water treatment, it requires expensive solvents and/or refrigeration equipment. Furthermore, expensive process vessels are necessary to handle the high operating pressures (about 1,000 psig) required by such liquid solvents.

Additionally, syngas is a combustible and toxic gas mixture, due primarily to its hydrogen (H₂) and carbon monoxide (CO) content. Consequently, syngas cleaning systems should be designed to ensure the safety of human personnel and equipment. While the specific safety mechanisms employed in syngas cleaning/handling systems are numerous, a key consideration is the pressure at which the system is operated. In negative-pressure systems, the syngas is generally handled and cleaned at a pressure below ambient atmospheric pressure (negative gauge pressure), while positive-pressure systems generally maintain the syngas above ambient atmospheric pressure (positive gauge pressure). While either approach offers certain operational and/or safety advantages, positive-pressure systems are often preferred from a safety standpoint because they prevent ingress of oxygen from the ambient air, which may result in the development of an explosive gas mixture within the system. While such systems are typically well-sealed, they still present the risk of syngas egress, which may result in unsafe levels of CO in the plant environment. However, careful equipment design, ventilation design, and the installation of CO monitors in working spaces can generally mitigate this risk.

Syngas treatment systems operating at positive gauge pressure are known in the art, including U.S. Pat. Nos. 7,056,487 and 6,090,356, described previously. In other industries, it is also common to handle combustible gases at positive gauge pressure. For example, U.S. Pat. No. 4,152,123 describes a water scrubber for removing particulates from high pressure blast furnace gas. While such systems offer the safety advantages inherent in positive pressure operation, they generally employ wet-based gas cleaning processes, expensive sorbents/solvents, expensive refrigeration processes, and/or technologies unproven at commercial scales.

It would be desirable to have a system for cleaning syngas which utilizes conventional and inexpensive gas cleaning equipment, while minimizing, and ideally eliminating, the need for waste water treatment of effluent streams. Additionally, it would be desirable to operate at least a portion of the system at a positive gauge pressure to improve safety.

SUMMARY

In one embodiment, a system is provided for processing a syngas stream including particulate matter, a combustible gas, and acid components. The system includes a gasifier vessel configured to produce a raw syngas stream; a gas cooling apparatus configured to cool the raw syngas stream to produce a cooled syngas stream; an HCl and particulate removal apparatus configured to remove at least a portion of HCl contained in the cooled syngas stream and at least a portion of remaining particulate matter in the cooled syngas stream to produce a reduced-HCl syngas stream; a first reheat apparatus configured to increase the temperature of the reduced-HCl syngas stream to produce a first reheated syngas stream; a COS and HCN hydrolysis apparatus configured to remove at least a portion of COS and HCN contained in the first reheated syngas stream to produce a hydrolyzed syngas stream; an H₂S removal apparatus configured to remove at least a portion of H₂S in the hydrolyzed syngas stream to produce a reduced-H₂S syngas stream; a second reheat apparatus configured to increase the temperature of the reduced- H₂S syngas stream to produce a second reheated syngas stream; an activated carbon bed apparatus configured to polish the second reheated syngas stream to produce a polished syngas stream; and a compression and intercooling apparatus configured to compress and cool the polished syngas stream to produce a clean syngas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

The single FIGURE is a process flow diagram of a syngas processing system.

DETAILED DESCRIPTION

In one aspect, the present invention relates to a syngas processing system that can be used in combination with gasifier reactors.

Syngas can be produced using various types of gasification reactors. For example, plasma gasification reactors (sometimes referred to as PGRs) are a type of pyrolytic reactor known and used for treatment of any of a wide range of materials including, for example, scrap metal, hazardous waste, other municipal or industrial waste and landfill material, and vegetative waste or biomass to derive useful material, e.g., metals, or a synthesis gas (syngas); or to vitrify undesirable waste for easier disposition. The reactor vessel can be used to process various feed material to produce a syngas that exits an upper portion of the reactor vessel. Syngas exiting the reactor typically requires subsequent processing to make it suitable for its intended use.

Various gasification reactor designs are known in the art. One example of a plasma gasification reactor is described in US Patent Application Publication US2012/0199795, which is incorporated by reference herein.

The single FIGURE is a process flow diagram of one example of a gasification reactor and an associated cleaning system 1. The major operational units of the system include a gasifier vessel 10; a gas cooling apparatus 20; a first particulate removal apparatus 30; a gas pressurization apparatus 40; a hydrochloric acid (HCl) and particulate removal apparatus 50; a first reheat apparatus 60; a carbonyl sulfide (COS) and hydrogen cyanide (HCN) hydrolysis apparatus 70; a hydrogen sulfide (H₂S) removal apparatus 80; an ammonia (NH₃) removal apparatus 90; a second reheat apparatus 100; an activated carbon bed apparatus 110; and a compression and intercooling apparatus 120. The various apparatus in the example system are arranged such that, in some cases, an upstream apparatus converts contaminants into compounds that can be collected in subsequent apparatus.

Additional features of the syngas cleaning system 1 include: water (or effluent) recirculation and discharge; particulate recycling and carbon utilization; and solid waste discharge.

A raw syngas stream 11 can be generated within a gasifier vessel 10 by the gasification of a carbonaceous feedstock stream. Gasification vessels that process carbonaceous feedstock are known in the art. Examples of carbonaceous feedstock include, but are not limited to: coal, petroleum, coke, biomass, municipal solid waste (MSW), refuse-derived fuel (RDF), industrial wastes, agricultural wastes, and sewage sludge. Additional input streams to the gasifier vessel 10 may be introduced, including but not limited to: an oxidant (e.g. oxygen, enriched air); steam; water; and/or an inert gas.

Additionally, the gasifier vessel 10 may generate a liquid slag output stream. In some examples, this stream may be granulated and cooled to produce an inert solid which may be sold as a commodity.

Energy may be supplied to the gasifier vessel 10 to ensure a sufficiently high operating temperature for the gasification reactions. In some systems, electrically-powered plasma torches may be employed to provide heat to the gasifier vessel 10. Other systems may operate without an external heating source.

In various embodiments, the raw syngas stream 11 is a gas mixture containing H₂ and CO. In some examples, the raw syngas stream 11 may also contain significant quantities of carbon dioxide (CO₂) and water vapor, as well as other gaseous and solid contaminants (e.g. particulate matter (PM)).

The raw syngas stream 11 can include numerous components including, for example, CO, CO₂, H₂O, O₂, N₂, Ar, H₂, Hydrocarbons, HCl, H₂S, COS, SO₂, NH₃, HCN, Hg, and/or Particulate Matter (PM). Significant variations in composition are possible depending on the specific gasification process and feedstock composition. It is possible that certain variations of the syngas components might need additional handling and cleaning operations.

The raw syngas stream 11 exiting the gasifier vessel 10 is at an elevated temperature. In some examples, the temperature of the raw syngas stream 11 may be approximately 850° C. to 1,150° C., although temperatures outside of this range are also possible (depending on the gasification process, feedstock characteristics, and/or other factors).

The raw syngas stream 11 can be cooled to a temperature suitable for the downstream handling and cleaning equipment. Cooling can be achieved by a gas cooling apparatus 20, located downstream of the gasifier vessel 10. Various types of the gas cooling apparatus 20 are possible, including but not limited to: evaporative spray cooling (wherein a water spray is introduced into the raw syngas stream 11, in order to cool the raw syngas stream 11 by evaporation of the water); a water-cooled duct (wherein heat from the raw syngas stream 11 is indirectly transferred to a water stream circulating within an annular plenum of a double-walled duct); a radiation-cooled duct (wherein heat from the raw syngas stream 11 is indirectly transferred to a metallic duct wall, and subsequently to an external environment); and/or a combination of water-cooled and radiation-cooled ducts.

Other cooling apparatus include an indirect heat exchanger (e.g. a forced-draft cooler, wherein the raw syngas stream 11 passes over a plurality of cooling tubes, and cool ambient air is passed through said cooling tubes, in order to extract heat from the raw syngas stream 11). In addition, cooling can be achieved by dilution with a lower-temperature syngas stream.

A cooled syngas stream 21 exits the gas cooling apparatus 20. The cooling apparatus can be designed such that the temperature of the cooled syngas stream 21 meets the requirements and limitations of the downstream processes and equipment. In some systems, the cooled syngas stream 21 may be at a temperature below 400° C. (an acceptable working temperature for mild steel). In other examples, the cooled syngas stream 21 may be at a temperature of 260° C. to enable efficient removal of certain contaminants (e.g. HCl removal in the downstream HCl and particulate removal apparatus 50, described below).

Depending on the composition of the raw syngas stream 11, it may be desirable to cool the raw syngas stream 11 as rapidly as possible, in order to minimize the formation of toxic dioxin and/or furan compounds (slow cooling generally promotes the formation of dioxins/furans). For example, some systems may employ evaporative spray cooling for the gas cooling apparatus 20 to minimize cooling time, and thus minimize the formation of dioxins/furans.

In embodiments of the gas cooling apparatus 20 that require water input (e.g. evaporative spray cooling), water may be supplied by a first recirculated water stream 22 (described below).

In some embodiments of the gas cooling apparatus 20, a portion of the particulate matter (PM) contained within raw syngas stream 11 may be removed by the gas cooling apparatus 20 and discharged as a first solids stream 23. This may be combined with a second solids stream 32 (described below) to form a solids recycle stream 12, which is then recirculated to the gasifier vessel 10. This PM recirculation process is further described below.

In the embodiment shown in the FIGURE, the cooled syngas stream 21 passes to a first particulate removal apparatus 30, which removes PM from the cooled syngas stream 21. A semi-clean syngas stream 31 (with a lower PM content compared to the cooled syngas stream 21) and a second solids stream 32 are discharged from the first particulate removal apparatus 30.

The second solids stream 32 may be combined with the first solids stream 23 to form the solids recycle stream 12, which is then recirculated to the gasifier vessel 10. In some systems, the first solids stream 23 may not be present, and the solids recycle stream 12 may consist entirely of the second solids stream 32.

PM removal in the first particulate removal apparatus 30 may be desirable for: (1) maximizing carbon utilization of the gasification process (e.g. PM in the cooled gas stream 21 may have an appreciable carbon content; and captured PM can be recycled to the gasifier vessel 10 to maximize carbon conversion to syngas); (2) minimizing the PM concentration passing to downstream equipment (e.g. in order to minimize PM build-up and reduce abrasion); and/or (3) capturing the PM in a dry form, in order to minimize discharge of PM-laden effluent streams.

Various PM removal technologies may be employed for the first particulate removal apparatus 30, including, but not limited to, a cyclone or a baghouse. Potential advantages and disadvantages of these two (2) technologies are summarized below. These advantages and disadvantages should not be considered as exhaustive and additional considerations may be required depending on the specific system of the syngas cleaning system 1.

Cyclone:

-   -   Minimal maintenance     -   Lower pressure drop compared to baghouses     -   No internals     -   Generally lower capital cost compared to baghouses     -   Low PM removal efficiency, especially with fine PM (<10 microns         in diameter). Some gasification processes (e.g. plasma MSW         gasification) may generate a significant portion of fine PM.

Baghouse:

-   -   High PM removal efficiency (outlet PM concentrations <10 mg/Nm³         are possible)     -   High removal efficiency for fine PM     -   Higher capital cost     -   Higher pressure drop     -   Requires careful temperature control in upstream gas cooling         apparatus 20 to avoid damage to filter material     -   Inert gas will be required for filter cleaning

Finally, in some syngas cleaning systems, the first particulate removal apparatus 30 may not be required.

In the system shown in the FIGURE, the semi-clean syngas stream 31 exits the first particulate removal apparatus 30 and passes to a gas pressurization apparatus 40. The purpose of the gas pressurization apparatus 40 is to pressurize the semi-clean syngas stream 31, in order to generally convey the syngas through the remainder of the syngas cleaning system 1. A pressurized syngas stream 41 exits the gas pressurization apparatus 40.

In some embodiments of the syngas cleaning system, the gas pressurization apparatus 40 may be a centrifugal fan. In other examples, multiple centrifugal fans may be employed in series to provide a higher pressure rise to the semi-clean syngas stream 31. Depending on the PM concentration of the semi-clean syngas stream 31, the gas pressurization apparatus 40 may be designed with abrasion-resistant elements to enable handling of particulate-laden gas.

In some embodiments of the syngas cleaning systems, it may be desirable to maintain the internal environment at a positive gauge pressure. In some examples, the pressure of the pressurized syngas stream 41 may be controlled by the gas pressurization apparatus 40 such that all points between the gas pressurization apparatus 40 and the compression and intercooling apparatus 120 are maintained at a positive gauge pressure. Furthermore, in some examples, the gasifier vessel 10 may be operated at a sufficiently high gauge pressure such that all points between the gasifier vessel 10 and the gas pressurization apparatus 40 are maintained at a positive gauge pressure. It should be noted that in some embodiments of the syngas cleaning system, a negative gauge pressure may be preferred or unavoidable at certain locations within the system.

In certain systems, the flow rate and/or pressure rise through the gas pressurization apparatus 40 may be controlled by one or more control mechanisms (not shown in the FIGURE), including, but not limited to: inlet damper(s); outlet damper(s); variable-speed drive(s) (VSDs); and/or recirculation of a portion of the pressurized syngas stream 41 to the inlet of the gas pressurization apparatus 40.

Additionally, the control mechanisms for the gas pressurization apparatus 40 may be modulated based on various operating parameters of the syngas cleaning system, including, but not limited to: static pressure within the gasifier vessel 10, and/or static pressure at any other location within the syngas cleaning system.

Additionally, certain embodiments of the syngas cleaning system may employ an alternate arrangement, wherein the gas pressurization apparatus 40 is located upstream of the first particulate removal apparatus 30. Because the first particulate removal apparatus 30 may result in a significant pressure reduction in the syngas stream, such a configuration may be utilized to ensure that the syngas stream remains at a positive gauge pressure. Such systems will expose the gas pressurization apparatus 40 to a higher PM loading, and additional design measures (e.g. abrasion resistance) may be required in these cases.

In the embodiment shown in the FIGURE, the pressurized syngas stream 41 passes to an HCl and particulate removal apparatus 50 to remove at least a portion of the HCl and/or PM contained within the pressurized syngas stream 41. The HCl and particulate removal apparatus 50 may include a circulating dry scrubber (CDS), wherein HCl is reacted with hydrated lime (Ca(OH)₂) according to the following chemical equation:

Ca(OH)₂+2HCl fi CaCl₂+2H₂O   [Equation 1].

CDS technology is widely described by existing art, including U.S. Pat. No. 8,715,600 and US Patent Application Publication No. 2013/0294992, so the proceeding description does not detail the specific sub-components of the HCl and particulate removal apparatus 50. Furthermore, some embodiments may utilize other conventional technologies for the HCl and particulate removal apparatus 50, such as a spray dryer absorber in conjunction with a baghouse. The HCl and particulate removal apparatus 50 may include the following operations:

1. Introduction of the pressurized syngas stream 41 into the HCl and particulate removal apparatus 50.

2. Introduction of at least one of the following water streams into the HCl and particulate removal apparatus 50:

-   -   a. A second recirculated water stream 52 (described below)     -   b. A first scrubber recirculation stream 85 (described below),         and/or     -   c. A second scrubber recirculation stream 95 (described below).

3. Introduction of a hydrated lime stream 53 into the HCl and particulate removal apparatus 50.

4. Evaporation of any water streams introduced into the HCl and particulate removal apparatus 50, by heat exchange with the pressurized syngas stream 41.

5. Neutralization of at least a portion of the HCl contained within the pressurized syngas stream 41, generally according to Equation 1.

6. Extracting a spent solids stream 54, containing solid products of the reaction of Equation 1, unreacted hydrated lime, reactants/byproducts contained in the first scrubber recirculation stream 85 and the second scrubber recirculation stream 95, and/or PM removed from the pressurized syngas stream 41.

7. Discharging a low-HCl syngas stream 51 through the gas outlet of the HCl and particulate removal apparatus 50.

Those skilled in the art will recognize that additional steps internal to the HCl and particulate removal apparatus 50 may be required depending on the specific technology employed.

The HCl and particulate removal apparatus 50 may be designed and operated to meet one or more of the following objectives:

-   -   Sufficient HCl removal (the required HCl removal efficiency can         depend on the inlet conditions and end-use of the syngas);     -   Sufficient PM removal (the required PM removal efficiency can         depend on the end-use of the syngas);     -   Complete evaporation of all input water streams;     -   Maintenance of the low-HCl syngas stream 51 at a temperature         above its water dew point (to prevent any moisture         condensation); and/or     -   Removal of at least a portion of the contaminants contained         within the first scrubber recirculation stream 85 and/or the         second scrubber recirculation stream 95 via the spent solids         stream 54, to prevent accumulation of these contaminants within         the syngas cleaning system.

In the embodiment shown in the FIGURE, the low-HCl syngas stream 51 (also referred to as a reduced-HCL syngas stream) passes to a first reheat apparatus 60, wherein the low-HCl syngas stream 51 is heated to produce a first reheated syngas stream 61. This operation may be included to ensure a sufficiently high syngas temperature at the inlet of the downstream COS and HCN hydrolysis apparatus 70. In some systems, the first reheated syngas stream 61 may be heated to a temperature of 250° C. In other systems, the low-HCl syngas stream 51 may be at a sufficiently high temperature to obviate the need for the first reheat apparatus 60.

The first reheat apparatus 60 (if required) may heat the low-HCl syngas stream 51 by various direct or indirect heat exchange mechanisms, including, but not limited to:

-   -   Indirect heat transfer with a higher temperature syngas stream         (e.g. the raw syngas stream 11);     -   Indirect heat transfer using a fuel-fired heater (possible fuels         include natural gas, cleaned syngas); or     -   Direct heat exchange by duct burners (possible fuels include         natural gas, cleaned syngas).

In some embodiments, an indirect heat transfer mechanism may be preferred to prevent dilution of the syngas stream and to maximize its heating value.

In the embodiment shown in the FIGURE, the first reheated syngas stream 61 passes to a COS and HCN hydrolysis apparatus 70, wherein at least a portion of the COS within the first reheated syngas stream 61 is converted to H₂S and CO₂, and at least a portion of the HCN within the first reheated syngas stream 61 is converted to NH₃ and CO, according to the following chemical equations:

COS+H₂O fi H₂S+CO₂   [Equation 2]

HCN+H₂O fi NH₃+CO   [Equation 3]

The above reactions generally occur in the presence of a catalyst within a packed bed vessel. Potential catalysts include, but are not limited to: alumina-based catalysts; chromia-alumina-based catalysts; or copper-chromia-alumina-based catalysts.

A hydrolyzed syngas stream 71 exits the COS and HCN hydrolysis apparatus 70.

The allowable composition, temperature, and pressure of the first reheated syngas stream 61 can depend on the specific technology employed for the COS and HCN hydrolysis apparatus 70. Potential considerations include:

-   -   Inlet gas temperature (for example, an inlet temperature of at         least 250° C. may be required for certain alumina-based         catalysts);     -   Inlet PM concentration (generally, this should be as low as         possible to prevent blockage of the packed bed and to improve         bed life);     -   Inlet heavy metals concentration (certain heavy metals, such as         mercury, may act as catalyst poisons and reduce the efficiency         of the hydrolysis reactions); and/or     -   Inlet acid gas concentration (certain acid gases, such as HCl,         may act as catalyst poisons and reduce the efficiency of the         hydrolysis reactions).

The COS and HCN hydrolysis apparatus 70 can convert COS and HCN into species which are more readily removed by the downstream syngas cleaning equipment.

Finally, the accumulation of contaminants within the COS and HCN hydrolysis apparatus 70 may eventually require replacement of the bed material. Consequently, used bed material may be periodically removed via a hydrolysis discharge stream 72 for disposal.

In the embodiment shown in the FIGURE, the hydrolyzed syngas stream 71 passes to an H₂S removal apparatus 80, which can include a wet-based scrubbing vessel in which at least a portion of the H₂S is removed from the hydrolyzed syngas stream 71 by neutralization with sodium hydroxide (NaOH), according to the following chemical equations:

H₂S+NaOH fi NaHS+H₂O   [Equation 4]

H₂S+2NaOH fi Na₂S+2H₂O   [Equation 5]

Additionally, the H₂S removal apparatus 80 may remove a portion of the HCl in the hydrolyzed syngas stream 71, according to the following chemical equation:

HCl+NaOH fi NaCl+H₂O   [Equation 6]

While the above reaction may proceed to some extent in various embodiments of the present invention, it can be appreciated that the H₂S removal apparatus 80 is not specifically intended to remove HCl from the hydrolyzed syngas stream 71; and HCl removal can be primarily achieved within the HCl and particulate removal apparatus 50. In some embodiments, it may be desirable to minimize the HCl content in the hydrolyzed syngas stream 71 in order to minimize consumption of NaOH by reaction with HCl within the H₂S removal apparatus 80.

Some embodiments of the H₂S removal apparatus 80 may utilize other reagents in combination with, or as alternatives to, NaOH. For example, it may be desirable to utilize sodium hypochlorite (NaOCl) in conjunction with NaOH.

Various designs for the wet-based scrubbing vessel are possible, including, but not limited to: spray chambers; packed-bed scrubbers; multi-vessel systems; and venturi scrubbers.

The H₂S removal apparatus 80 can be supplied with a third recirculated water stream 82 and a sodium hydroxide solution stream 83. In the embodiment shown in the FIGURE, a low-H₂S syngas stream 81 (also referred to as a reduced-H₂S syngas stream) exits the H₂S removal apparatus 80 with a lower concentration of H₂S than the hydrolyzed syngas stream 71. Some embodiments may incorporate internal recirculation of any scrubbing fluid(s) within the H₂S removal apparatus 80 to maximize utilization of the scrubbing reagent(s).

In the embodiment shown in the FIGURE, a first scrubber blowdown stream 84 is discharged from the H₂S removal apparatus 80. A portion of the first scrubber blowdown stream 84 may be recirculated to the HCl and particulate removal apparatus 50 as a first scrubber recirculation stream 85. A portion of the first scrubber blowdown stream 84 may also be discharged as a first scrubber discharge stream 86 (e.g. for waste-water treatment). The flow rates of the first scrubber recirculation stream 85 and the first scrubber discharge stream 86 may vary depending on the specific embodiment. In some examples, it may be desirable to maximize the flow rate of the first scrubber recirculation stream 85 in order to minimize effluent output from the syngas cleaning system. However, it can be understood that at least a small flow rate of the first scrubber discharge stream 86 may be required to avoid accumulation of byproducts from the H₂S removal apparatus 80 due to recirculation within the syngas cleaning system.

In the embodiment shown in the FIGURE, the low-H₂S syngas stream 81 passes to an NH₃ removal apparatus 90, which comprises a wet-based scrubbing vessel in which a portion of the NH₃ is removed from the low-H₂S syngas stream 81 by neutralization with a sulfuric acid (H₂SO₄) solution, according to the following chemical equation:

2NH₃+H₂SO₄ fi (NH₄)₂SO₄   [Equation 7]

Various designs for the wet-based scrubbing vessel are possible, and may include, but are not limited to, the types described herein.

In the embodiment shown in the FIGURE, the NH₃ removal apparatus 90 is supplied with a fourth recirculated water stream 92 and a sulfuric acid solution stream 93. A low-NH₃ syngas stream 91 (also referred to as a reduced-NH₃ syngas stream) exits the NH₃ removal apparatus 90 with a lower concentration of NH₃ than the low-H₂S syngas stream 81. Other embodiments may incorporate internal recirculation of any scrubbing fluid(s) within the NH₃ removal apparatus 90 to maximize utilization of the scrubbing reagent(s).

In the embodiment shown in the FIGURE, a second scrubber blowdown stream 94 is discharged from the NH₃ removal apparatus 90. A portion of the second scrubber blowdown stream 94 may be recirculated to the HCl and particulate removal apparatus 50 as a second scrubber recirculation stream 95. A portion of the second scrubber blowdown stream 94 may also be discharged as a second scrubber discharge stream 96 (e.g. for waste-water treatment). The flow rates of the second scrubber recirculation stream 95 and the second scrubber discharge stream 96 may vary depending on the specific embodiment. In some examples, it may be desirable to maximize the flow rate of the second scrubber recirculation stream 95 in order to minimize effluent output from the syngas cleaning system. However, it should be understood that at least a small flow rate of the second scrubber discharge stream 96 may be used to avoid accumulation of byproducts from the NH₃ removal apparatus 90 due to recirculation within the syngas cleaning system.

In some embodiments (e.g. where the syngas is to be used for certain gas turbine applications), the NH₃ removal apparatus 90 may not be required. Depending on the particular application, the NH₃ concentration in the low-H₂S syngas stream 81 may be sufficiently low to preclude any dedicated NH₃ removal equipment. This may be possible in, for example, sites with sufficiently high NH₃ emission limits, or in gas turbine applications where post-combustion NO_(x) control technology is capable of handling a sufficiently high NH₃ content in the uncombusted syngas.

In the embodiment shown in the FIGURE, the low-NH₃ syngas stream 91 passes to a second reheat apparatus 100, wherein the low-NH₃ syngas stream 91 is heated to produce a second reheated syngas stream 101.

Due to the wet-based scrubbing processes employed for the upstream H₂S removal apparatus 80 and NH₃ removal apparatus 90, the low-NH₃ syngas stream 91 may be saturated with water. In order to avoid water condensation (and subsequent plugging/fouling) within the downstream activated carbon bed apparatus 110, it may be desirable to maintain the second reheated syngas stream 101 at a temperature above the water dew point. The preferred temperature for the second reheated syngas stream 101 may generally be determined by the acceptable operating temperature range of the activated carbon bed apparatus 110. In certain embodiments, the second reheated syngas stream 101 may be at a temperature of 120° C.

As with the first reheat apparatus 60, the second reheat apparatus 100 may heat the low-NH₃ syngas stream 91 by various direct or indirect heat exchange mechanisms, including, but not limited to, the mechanisms listed above.

In some systems, an indirect heat transfer mechanism may be preferred to prevent dilution of the syngas stream and to maximize its heating value.

Additionally, in some embodiments, the first reheat apparatus 60 may be interconnected and/or combined with the second reheat apparatus 100. Such an arrangement may be desirable for various reasons. For example, it may enable utilization of a common heat source for both reheat apparatuses, which may reduce equipment and/or heating fuel costs.

In the embodiment shown in the FIGURE, the second reheated syngas stream 101 passes to an activated carbon bed apparatus 110, wherein at least a portion of the remaining contaminants in the second reheated syngas stream 101 may be removed. A polished syngas stream 111 exits the activated carbon bed apparatus 110.

The activated carbon bed apparatus 110 can comprise a packed bed of activated carbon sorbent. Depending on the system, the activated carbon sorbent may be impregnated with various compounds (e.g. an acid-gas neutralizing compound) in order to enable further removal of other contaminants. While the activated carbon bed apparatus 110 may be employed primarily for mercury (Hg) removal, it may also enable removal of other contaminants, including, but not limited to: H₂S; HCl; COS; HCN; NH₃; dioxins/furans; and/or other heavy metals.

The specific sorbent employed in the activated carbon bed apparatus 110 may depend on the particular system. That is, specific sorbent may be selected based on the end-use application for the syngas and the contaminant levels in the second reheated syngas stream 101.

In other embodiments, the activated carbon bed apparatus 110 may be located downstream of the compression and intercooling apparatus 120. Such systems may be preferred in order to reduce the overall size and cost of the activated carbon bed apparatus 110, as the actual volumetric flow rate of the inlet syngas stream would be significantly lower due to its higher pressure and reduced concentration of water vapor. It should be noted that such a configuration would require additional reinforcement of the activated carbon bed apparatus 110 in order to handle the higher operating pressure. Furthermore, because at least a portion of the condensate from the compression and intercooling apparatus 120 is recirculated back into the syngas cleaning system 1 (i.e. as the condensate recirculation stream 124, also called a clean water stream), it may be desirable to maximize the amount of contaminants removed upstream of the compression and intercooling apparatus 120. In such cases, it may be preferable to locate the activated carbon bed apparatus 110 upstream of the compression and intercooling apparatus 120, as shown in the FIGURE.

Additionally, the accumulation of contaminants within the activated carbon bed apparatus 110 may eventually require replacement of the bed material. Consequently, used bed material may be periodically removed via an activated carbon bed discharge stream 112 for disposal.

In the embodiment shown in the FIGURE, the polished syngas stream 111 passes to a compression and intercooling apparatus 120, wherein the syngas is pressurized to a pressure required for an end-use of the syngas. In systems employing syngas for gas turbine combustion, a pressure of approximately 3,000 kPa(g) may be preferred (although other pressures are possible depending on the particular gas turbine equipment). A clean syngas stream 121 is discharged from the compression and intercooling apparatus 120.

The compression and intercooling apparatus 120 can generally comprise one or more stages of compressors. Each stage may contain multiple compressors arranged in parallel. Between each compressor stage, the syngas may be cooled by intercoolers (e.g. shell and tube heat exchangers). The compression and intercooling processes will condense at least a portion of the water vapor contained within the syngas, so each intercooling stage may be followed by droplet knockout and mist elimination equipment in order to remove any liquid water from the syngas stream. Water condensed within the compression and intercooling apparatus 120 can be discharged as a condensate stream 122. The condensate stream 122 may contain dissolved contaminants originating from the polished syngas stream 111, including, but not limited to: NH₃; CO₂; H₂S; HCN; and/or Hg.

It may be desirable to minimize the contaminant levels in the polished syngas stream 111 in order to: minimize the amount of contaminants in the condensate stream 122, thereby minimizing requirements for any downstream water treatment steps; and/or reduce corrosion within the compression and intercooling apparatus 120 by condensed acid species (e.g. H₂S).

In some embodiments, corrosion-resistant materials of construction (e.g. stainless or duplex steel tubes in the intercooler and interconnecting pipework) may be used to handle these acid species.

In the embodiment shown in the FIGURE, the condensate stream 122 from the compression and intercooling apparatus 120 is divided into a condensate discharge stream 123 and a condensate recirculation stream 124. The condensate discharge stream 123 is discharged from the syngas cleaning system, and may be sent to a downstream waste water treatment process.

The condensate recirculation stream 124 may be divided into the following streams:

-   -   a first recirculated water stream 22;     -   a second recirculated water stream 52;     -   a third recirculated water stream 82; and     -   a fourth recirculated water stream 92.

As described in above, certain embodiments of the gas cooling apparatus 20 may include water input (e.g. evaporative spray cooling). This water input may be provided by the first recirculated water stream 22. Other systems of the gas cooling apparatus 20 may not require any water input (e.g. a radiation-cooled duct). In these examples, the flow rate of the first recirculated water stream 22 may be zero.

As described above, the HCl and particulate removal apparatus 50 may be supplied by at least one of the following water streams:

-   -   the second recirculated water stream 52;     -   the first scrubber recirculation stream 85; and/or     -   the second scrubber recirculation stream 95.

In some embodiments, the flow rates of the first scrubber recirculation stream 85 and the second scrubber recirculation stream 95 may not be sufficient to meet the water input requirements of the HCl and particulate removal apparatus 50. In these cases, the balance may be provided by the second recirculated water stream 52. In other systems, the second recirculated water stream 52 may not be required, and its flow rate may be zero.

In some embodiments, the water input required for the H₂S removal apparatus 80 may be provided by the third recirculated water stream 82.

In some embodiments, the water input required for the NH₃ removal apparatus 90 may be provided by the fourth recirculated water stream 92.

It can be appreciated that the various water recirculation steps described above may minimize the amount of liquid effluent discharge from the syngas cleaning system 1, due to the following processes:

-   -   recirculation of blowdown from the H₂S removal apparatus 80 (as         the first scrubber recirculation stream 85) to the HCl and         particulate removal apparatus 50, and subsequent evaporation of         the water in the first scrubber recirculation stream 85 within         the HCl and particulate removal apparatus 50;     -   recirculation of blowdown from the NH₃ removal apparatus 90 (as         the second scrubber recirculation stream 95) to the HCl and         particulate removal apparatus 50, and subsequent evaporation of         the water in the second scrubber recirculation stream 95 within         the HCl and particulate removal apparatus 50; and/or     -   recirculation of condensed water from the compression and         intercooling apparatus 120 to one or more locations within the         syngas cleaning system 1, specifically:         -   the gas cooling apparatus 20;         -   the HCl and particulate removal apparatus 50;         -   the H₂S removal apparatus 80; and/or         -   the NH₃ removal apparatus 90.

Generally, some water discharge from the syngas cleaning system 1 may be required, because the total water contained within the raw syngas stream 11 will usually be higher than the total water content within the clean syngas stream 121. As evident in embodiment shown in the FIGURE, water may be discharged from the syngas cleaning system 1 via the following streams:

-   -   the first scrubber discharge stream 86;     -   the second scrubber discharge stream 96; and/or     -   the condensate discharge stream 123.         It can be appreciated that, for a given amount of total water         discharged from the syngas cleaning system 1, it may be         desirable to maximize the portion discharged via the condensate         discharge stream 123, as the contaminant levels within this         water stream (and thus water treatment requirements) will         generally be lower than either the first scrubber discharge         stream 86 or the second scrubber discharge stream 96. In some         embodiments, it may be desirable to minimize the flow rates of         the first scrubber discharge stream 86 and the second scrubber         discharge stream 96. For example, in some embodiments, the flow         rate of the first discharge stream 86, or the second discharge         stream, or both may be zero.

A portion of the PM contained within the raw syngas stream 11 may be captured in the gas cooling apparatus 20 (as the first solids stream 23) and in the first particulate removal apparatus 30 (as the second solids stream 32). Either (or both) of these streams may be recirculated back to the gasifier vessel 10 as the solids recycle stream 12.

The PM within the raw syngas stream 11 may contain some carbonaceous particulate, and it may be desirable to capture and recirculate at least a portion of this material to the gasifier vessel 10 for conversion to syngas. Such a recirculation process may increase the carbon utilization (i.e. the total amount of carbonaceous gasifier feed converted into syngas) of the gasifier vessel 10. Benefits of increased carbon utilization include, but are not limited to:

-   -   increased syngas production;     -   increased feedstock destruction (e.g. in the case of an MSW-fed         gasifier, which may be employed as a means of processing waste);         and/or     -   reduced solid waste discharge from the syngas cleaning system 1.

Various mechanisms for conveying the first solids stream 23, the second solids stream 32, and the solids recycle stream 12 to the gasifier vessel 10 are possible, including, but not limited to:

-   -   a gravity-operated mechanism (the feasibility of this mechanism         may depend on the particular layout of the syngas cleaning         system 1—generally, the gas cooling apparatus 20 and the first         particulate removal apparatus 30 would be at a higher elevation         than the gasifier vessel 10); and/or     -   pneumatic conveying (this mechanism may require an inert gas to         convey the PM within the streams, in order to avoid introduction         of oxygen into the syngas cleaning system, and the consequent         explosion hazard).

Solid waste from the syngas cleaning system can be discharged from the HCl and particulate removal apparatus 50 (as the spent solids stream 54) and, periodically, from the COS and HCN hydrolysis apparatus 70 and the activated carbon bed apparatus 110.

At least one liquid stream may be recirculated to the HCl and particulate removal apparatus 50, specifically:

-   -   the second recirculated water stream 52;     -   the first scrubber recirculation stream 85; and/or     -   the second scrubber recirculation stream 95.

These liquid streams may contain contaminants (referred to as recirculated contaminants), including byproducts from the H₂S removal apparatus 80, byproducts from the NH₃ removal apparatus 90, and contaminants originally present in the raw syngas stream 11. In some embodiments, a portion of these recirculated contaminants may be captured by the HCl and particulate removal apparatus 50 and discharged via the spent solids stream 54.

It can therefore be understood that the HCl and particulate removal apparatus 50 may effectively serve as a solid-liquid separator, in the sense that at least a portion of the contaminants contained within any incoming liquid stream(s) may be captured and discharged in a solid form, while the water contained within any incoming liquid stream(s) may be evaporated.

For low-effluent operation, it can also be understood that preferred systems can maximize the amount of recirculated contaminants removed within the HCl and particulate removal apparatus 50. This will minimize the contaminant concentration and/or flow rate of any liquid discharge streams from the syngas cleaning system 1.

The syngas cleaning systems described herein may seek to minimize overall capital and operating costs by enabling the use of relatively inexpensive technology for certain operations. The syngas cleaning/handling system can be used to produce a syngas stream suitable for its intended end-use. Potential end-uses are numerous, and include power generation (e.g. via syngas combustion in a gas turbine) and synthesis of liquid fuels. The final temperature, pressure, and contaminant levels of a syngas stream will be dictated by the specific end-use.

Various embodiments of the syngas cleaning systems described herein may seek to achieve one or more of the following objectives:

-   -   Minimization of any effluent discharge,     -   Minimization of contaminant levels within any effluent         discharge,     -   Opportunities for dry dust capture and recirculation to the         gasifier,     -   Minimization of fresh water input requirements (i.e. by         recirculation of liquid streams within the system),     -   Optimization of reagent consumption (e.g. by employing hydrated         lime reagent for HCl removal and sodium hydroxide reagent for         H₂S removal, in order to limit the amount of (costlier) sodium         hydroxide consumed for HCl removal),     -   Maintenance of a positive gauge pressure throughout at least a         portion of the system (which may be desirable from a safety         standpoint), or     -   Minimization of capital cost (by enabling use of conventional         and relatively cheap equipment for each unit operation).

The embodiment shown in the FIGURE includes numerous apparatus and connections between such apparatus. However, various embodiments of the invention defined in the claims need not include all of the apparatus shown in the FIGURE. For example, one embodiment may include a gas treatment system apparatus having a gasifier vessel configured to produce a gas stream including particulate matter, a combustible gas, and acid components and a gas treatment apparatus configured to receive the gas stream, wherein the gas treatment apparatus includes a dry acid gas removal apparatus.

Another embodiment may include an apparatus having a gasifier vessel configured to produce a combustible gas stream including particulate matter, a combustible gas, and acid components and a gas treatment apparatus configured to receive the gas stream, and remove contaminants therein, wherein the gas treatment apparatus operates at a positive gauge pressure.

Another embodiment may include apparatus having a gasifier vessel configured to produce a gas stream including particulate matter, a combustible gas, and acid components, and a gas treatment apparatus configured to receive the gas stream and remove contaminants therein, wherein the gas treatment apparatus includes a dry acid gas removal device and operates at a positive gauge pressure.

The embodiment shown in the FIGURE was designed to meet the requirements for gas turbine combustion applications. However, it should be understood that the syngas cleaning systems described above do not preclude any particular syngas end-use. Clean syngas requirements for liquid fuels synthesis are, generally, significantly more stringent that those for gas turbine combustion.

While particular aspects of the invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the disclosed system may be made without departing from the invention as defined in the appended claims. 

What is claimed is:
 1. A system for processing a syngas stream including particulate matter, a combustible gas, and acid components, the system comprising: a gasifier vessel configured to produce a raw syngas stream; a gas cooling apparatus configured to cool the raw syngas stream to produce a cooled syngas stream; an HCl and particulate removal apparatus configured to remove at least a portion of HCl contained in the cooled syngas stream and at least a portion of remaining particulate matter in the cooled syngas stream to produce a reduced-HCl syngas stream; a first reheat apparatus configured to increase the temperature of the reduced-HCl syngas stream to produce a first reheated syngas stream; a COS and HCN hydrolysis apparatus configured to remove at least a portion of COS and HCN contained in the first reheated syngas stream to produce a hydrolyzed syngas stream; an H₂S removal apparatus configured to remove at least a portion of H₂S in the hydrolyzed syngas stream to produce a reduced-H₂S syngas stream; a second reheat apparatus configured to increase the temperature of the reduced- H₂S syngas stream to produce a second reheated syngas stream; an activated carbon bed apparatus configured to polish the second reheated syngas stream to produce a polished syngas stream; and a compression and intercooling apparatus configured to compress and cool the polished syngas stream to produce a clean syngas stream.
 2. The system of claim 1, further comprising: a first particulate removal apparatus configured to remove particulates from the cooled syngas stream prior entry of the cooled gas stream to the HCl and particulate removal apparatus.
 3. The system of claim 2, further comprising a path configured to return at least a portion of the particulate matter removed in the first particulate removal apparatus to the gasifier vessel.
 4. The system of claim 2, further comprising: a gas pressurization apparatus configured to pressurize the cooled syngas gas stream prior to the first particulate removal apparatus.
 5. The system of claim 2, further comprising: a gas pressurization apparatus configured to pressurize the cooled syngas stream after the first particulate removal apparatus.
 6. The system of claim 1, wherein the compression and intercooling apparatus is configured to extract water from the polished syngas stream to produce a clean water return stream and to increase the pressure of the clean syngas stream.
 7. The system of claim 6, further comprising: a clean water return from the compression and intercooling apparatus to at least one of the gas cooling apparatus, the HCl and particulate removal apparatus, and the H₂S removal apparatus.
 8. The system of claim 1, wherein the syngas cooling apparatus is configured to remove at least a portion of the particulate matter within the raw syngas stream.
 9. The system of claim 8, further comprising a path configured to return at least a portion of the particulate matter removed in the syngas cooling apparatus to the gasifier vessel.
 10. The system of claim 1, wherein the HCl and particulate removal apparatus is configured to discharge a dry waste stream.
 11. The system of claim 1, wherein the H₂S removal apparatus is configured to generate a first effluent return stream, and the first effluent return stream is returned to the HCl and particulate removal apparatus.
 12. The system of claim 11, wherein all water within the first effluent return stream is evaporated in the HCl and particulate removal apparatus, and at least a portion of the contaminants within the first effluent return stream is removed by the HCl and particulate removal apparatus.
 13. The system of claim 1, wherein the H₂S removal apparatus is configured to generate a first effluent discharge stream.
 14. The system of claim 13, wherein a flow rate of the first effluent discharge stream is zero.
 15. The system of claim 1, wherein the second reheat apparatus is configured to increase the temperature of the syngas to above a dew point temperature.
 16. The system of claim 1, wherein the activated carbon bed apparatus is configured to remove at least a portion of the mercury contained within the syngas.
 17. The system of claim 1, wherein the first reheat apparatus and the second reheat apparatus are connected to a common heat source.
 18. The system of claim 1, further comprising: an NH₃ removal apparatus configured to remove at least a portion of the NH₃ within the reduced-H₂S syngas stream.
 19. The system of claim 18, further comprising: a clean water return from the compression and intercooling apparatus to at least one of the gas cooling apparatus, the HCl and particulate removal apparatus, the H₂S removal apparatus, and the NH₃ removal apparatus.
 20. The system of claim 18, wherein the NH₃ removal apparatus is configured to generate a second effluent return stream and the second effluent return stream is returned to the HCl and particulate removal apparatus.
 21. The system of claim 20, wherein all water within the second effluent return stream is evaporated in the HCl and particulate removal apparatus, and at least a portion of contaminants within the second effluent return stream is removed by the HCl and particulate removal apparatus.
 22. The system of claim 21, wherein a flow rate of second effluent discharge stream is zero.
 23. The system of claim 18, wherein the NH₃ removal apparatus is configured to generate a second effluent discharge stream. 